iwlwifi: mvm: add support for 32kHz external clock indication

iwlwifi: mvm: add support for 32kHz external clock indication

In low power modes, the chip clock source for platform integrated
devices is 32kHz. It is generated internally and supplied by a crystal
oscillator. However using a 32kHz sourced from crystal oscillator
has high power penalty.

There is an option to get an external 32kHz clock from the platform. Past
experience shows that the reliability is platform dependent,
i.e. on some platforms it works good and on other it doesn’t.

Working from external clock will save 0.5 mW in sleep state, from overall
1.8mW that we have today, i.e. almost 30%.

Each OEM can enable or disable the use of the external 32kHz clock by
setting a BIOS configuration. In case the OEM configured to use 32kHz
external clock the driver will pass this indication to the FW.

Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>